The individual electrodes utilized in an electrostatic print head are very small and large numbers of them are required to give desired resolution on a given length of line. There may be 1728 of them, for example, for printing on an A4 format medium at 8 points per millimeter.
In practical embodiments, in order to avoid applying the high tension required to give an ion discharge for printing an electrostatic image solely to the electrodes, the electrodes are associated with a counter-electrode which is also raised to high tension. Under such conditions, the high tension required for ion discharge is applied to each electrode/counter-electrode pair with the high tension that is applied to the electrode on its own or to the counter-electrode on its own being less than a threshold value for causing ion discharge, and thus being incapable of printing.
In direct electrostatic printing, special paper is used comprising a base of conductive paper covered with a dielectric deposit which is a few microns thick, such paper is called dielectric paper and a latent electrostatic image can be printed directly thereon. The latent image is then inked or developed by means of a magnetic brush or by any other developer means, and the developed image is fixed by pressure or in an oven.
In indirect electrostatic printing, dielectric paper is not used, but an intermediate recording medium is used, such as a dielectric deposit on a drum or simply a thin insulating film (e.g. 10 .mu.m to 20 .mu.m thick), and ordinary paper, preferably in sheets, is used as a final medium. The latent electrostatic image is created on the intermediate medium, and then inked by means similar to those used for direct electrostatic printing, and the inked image is then transferred, e.g. by pressure or by corona, to a sheet of ordinary paper whee it is fixed.
In direct electrostatic printing, because of the possibility of conduction through the conducting layer of the dielectric paper used, and to minimize the number of high tension transistors needed to power the electrodes, the electrodes are divided into identical groups, an independent counter-electrode of substantially the same length as a group of electrodes is attributed to each group of electrodes, and electrode power supply demultiplexing is provided by interconnecting all the electrodes occupying the same positions in the various groups. Thus, if the printing high tension has the value V, a voltage of V/2 is applied to the counter-electrode attributed to one of the groups of electrodes while the other counter-electrodes are at a potential of 0 volts, and a "printing" voltage of -V/2 or of 0 volts (depending on whether a mark is to be printed or not) is applied to the electrodes of the various groups with printing being possible only for the electrodes in the group associated with the counter-electrode at V/2. A complete line is thus printed on a recording medium by sequentially powering different position electrodes in as many successive cycles as there are groups of electrodes, and in powering a single respective counter-electrode for each group cycle.
For a print head having 1728 electrodes, an optimal arrangement is defined by having 36 groups of 48 electrodes each, giving a total of 84 power switches.
In commonly used direct electrostatic printing systems, and in particular in high resolution systems, printing problems could be associated with the electrodes opposite the gaps between counter-electrodes where the electric field is of reduced strength in spite of the conductivity of the conductive layer of the dielectric paper. The gaps are 0.1 to 0.5 mm wide. These problems are avoided by disposing each counter-electrode of length substantially equal to the length of a group of electrodes, opposite electrodes belonging to two successive groups. These is thus one more counter-electrode than there are groups of electrodes and the set of counter-electrodes overlaps from both ends of the line of electrodes, In other words each of the end counter-electrodes is disposed opposite to a part only of the corresponding end group of electrodes. The demultiplexing circuit then used interconnects the electrodes in corresponding positions in even numbered groups into a first network and interconnects the electrodes in the same positions in odd numbered groups into a second and independent network. Both networks are connected to as many individual power switches as there are electrodes in a group and each counter-electrode is also connected to an individual power switch. Printing is then performed by applying the printing voltage of -V/2 or 0 volts to the electrodes in successive positions in each network alternately, while at the same time applying the voltage of V/2 simultaneously to the two successive counter-electrodes which are disposed opposite the current group of electrodes, thereby minimizing the side effects due to the gaps between adjacent counter-electrodes.
Further, in high resolution printing systems, problems due to the small electrode pitch are avoided by arranging the electrodes in two identical and independent rows which are offset relative to one another by half the pitch of the electrodes along each row and in which the electrodes are associated by group. The counter-electrodes are then associated with the electrodes of both rows, and overlap on either side of the rows.
In currently used direct electrostatic printing systems, the electrodes and the counter-electrodes associated therewith and in the vicinity thereof, may be disposed either on opposite sides of the dielectric paper, or else on the same side of the dielectric paper opposite the face with the dielectric deposit, with two identical rows of counter-electrodes then being used regardless of whether there are one or two rows of electrodes. The two rows of counter-electrodes are either disposed on either side of a single row of electrodes, or else they are disposed on either side of the set of both rows of electrodes with two facing counter-electrodes always being connected to the same potential.
Indirect electrostatic printing systems use a set or "comb" of electrodes identical or analogous to those used in direct electrostatic printing systems. The electrodes are applied against one face of a dielectric film which constitutes the intermediate recording medium, and a single counter-electrode is applied to the other face of the film opposite the comb of electrodes. The absence of a conductive layer in the intermediate support makes it impossible to associate independent counter-electrodes with the electrodes of the comb, since printing cannot take place between the counter-electrodes, and it is consequently impossible to use a demultiplexer circuit.
Preferred embodiments of invention enable a demultiplexer circuit to be used in an electrostatic print head in such a manner that the head can perform direct or indirect electrostatic printing on a conventional recording medium.